Traditionally on large subsonic passenger aircraft, metallic or composite upper panels cover the trailing edge of the wing, aft of the rear spar. These panels are attached to the trailing edge ribs by bolts that pass through the panels and the trailing edge ribs. Each panel also requires an attachment to the wing cover (also known as a wing skin), traditionally via a series of butt straps that will connect both panel and wing cover. Each butt strap and panel will require multiple bolts.
This traditional arrangement brings a number of problems. Firstly, in order to minimise weight, both the wing cover and trailing edge ribs will be optimised and accordingly any variations in panel and rib thickness will be taken up by differing bolt grip lengths. All bolts will be countersunk and due to tolerancing there may be either a raised or recessed countersunk head that will contribute to overall aerodynamic drag. Secondly, the panels tend to introduce a span-wise and chord-wise step or discontinuity in the profile of the wing, increasing drag. Thirdly, there is a large part count and hence high manufacturing costs (on each panel there may be more than fifty bolts, and the number of panels may be in excess of twenty). Fourthly, each panel requires a significant amount of tooling and setting up time to ensure that the trailing edge ribs and the panels will be interchangeable between aircraft. Fifthly, each panel is typically manufactured from a carbon monolithic honeycomb construction to minimise the panel weight. The panel design is tailored to suit the air loads and accommodate stresses due to sympathetic wing bending. If the wing cover is metallic and the panel composite there will also be a requirement to consider differential thermal expansion between the different materials. Finally, the limited size and strength of the so-called “cover overhang” (that is, the portion of the cover which overhangs to the rear of the spar) means that it is not capable of reacting a significant amount of the load from aerodynamic control elements (such as ailerons or spoilers) mounted on the trailing edge of the wing.